Writable::Writer needs DerefMut trait bound

Otherwise, a call to `bits` will fail to typecheck.

Author: couchand

src/generate/generic.rs

@@ -21,7 +21,7 @@ pub trait Readable: RegisterSpec {
 /// Registers marked with `Readable` can be also `modify`'ed.
 pub trait Writable: RegisterSpec {
     /// Writer type argument to `write`, et al.
-    type Writer: core::convert::From<W<Self>> + core::ops::Deref<Target = W<Self>>;
+    type Writer: core::convert::From<W<Self>> + core::ops::DerefMut<Target = W<Self>>;
 }
 
 /// Reset value of the register.